Dec. 22, 1923 
Cytology of Wheat Stem Rust 
577 
also is increasing in quantity. This expansion continues (<i and e) 
until the volume is several times the normal and the thin delicate nuclear 
membrane incloses a large meshed chromatin net composed of very 
fine threads. The nucleolus is greatly enlarged. This condition is 
maintained for only a few days and is followed by the collapse and death 
of the nucleus (/ and g). This may begin on the tenth day but usually 
is not conspicuous until a little later. By the fourteenth day, however, 
the nucleus is flattened into a disk, usually lying against the cell wall. 
This decrease in volume is best seen in edge view ( h ). The nucleus is 
now a red stained body from which all trace of nuclear structure has 
been lost. 
To obtain more exact information as to the extent of the nuclear 
changes in an infection area and beyond it during the spread and devel¬ 
opment of the fungus, outline camera drawings were made of nuclei in 
the center of the infection and at graded distances from it. In each zone 
in infections 7, 10, and 14 days of age, 10 nuclei were drawn (X 1130) and 
both their long diameters and short diameters were measured and added. 
The figures in the tabulated results (Table I) are not in microns but are di¬ 
rectly comparable, as each represents the sum in millimeters of the long and 
the short diameters of 10 nuclei or 10 plastids at a magnification of 1130, 
The figures for the normal nucleus are about 115 by 70. In the maet- 
rial from seven-day infections the effect of the fungus is pronounced, the 
nuclear size at the center of the infection being 140 by 90. The effect of 
the fungus is seen only at the center of the infection in the earlier stages, 
but soon extends farther outward, keeping pace to some extent with the 
spread of the fungus itself. 
The maximum nuclear size is found in ten-day infections (Table I, A, 10a 
and 10b), where the nuclei in the vicinity of the young uredinium reach 
the dimensions 143 by 116 and 159 by 123, showing that the volume of 
the nucleus has increased several fold. There is less and less enlargement 
of the nuclei toward the margin of the infection area and at this stage 
there is little, if any, change from the normal nuclear condition beyond 
the range of the fungus itself. 
Four days later (14a), when spore formation is at its height, the host 
nucleus collapses, as is shown by the sharp decrease in the short diameter, 
which is now reduced from 100 or even more to about 40. Moreover, the 
dimensions of nuclei in the area which is beyond the fungus, 145 by 49, 
show that here, too, the nuclei undergo a tardy expansion and then 
collapse. 
It is of interest to note the effect where two uredinia occur just far 
enough apart so that their mycelia are tangent, or nearly so. In this case 
(Table I, B, nuclei, 10c, 14b) the nuclei expand in cells throughout the 
intermediate area between the two uredinia. 
A different relation exists between the fungus and the size of plastids 
in infected host tissue. Measurements here (Table I, plastids) were made 
from the same areas of the same infections that were used in the nuclear 
studies and the same methods were followed. 
In general the plastids within the infected region are markedly decreased 
in size. The numbers for the normal plastid (Table I, A, plastids, 2a) are 
60 by 25. One week after inoculation (7a and 7b) plastids throughout 
the infected area are reduced one-third or more, being approximately 40 
by 20. With few exceptions the plastids in the center of the infection 
show no further reduction, maintaining a fairly constant size somewhere 
near 40 by 20 until the uredinium is full grown. 
